[Boatanchors] DM-28 Dynamotor Restoration Questions

Thu Apr 9 10:45:53 EDT 2020

 Hi Scott,To use a switcher power supply you need a shielded power supply cable. You need cable with two conductors for power and a braided shield. The shield is grounded to the P/S case and the radio. Henry Rogers discusses this on the radioblvd web site. Look under restoring the ART-13 transmitter. In the power supply section he discusses cable shielding when using a switcher.
73, Mike, KE5YTV

    On Wednesday, April 8, 2020, 6:49:37 PM CDT, whitebear1122 at comcast.net <whitebear1122 at comcast.net> wrote:  

 Hi Mike and All,  Thanks for your comments.  I was both happy and surprised that the brush wire repair worked.  

Regarding the dynamotor noise question, the dynamotor/348 chassis is outside of the cabinet.  I don’t know what a ‘normal’ dynamotor whine should be like so I don’t have a clue if it’s abnormally loud or not.  Based on yours and others comments, I will take a search of YOUTUBE and look for dynamotor videos or rigs running a dynamotor to try and gauge the noise. 

No I did not replace the bearings.  I did not know there was such an option.  I took the dynamotor apart to re-grease the bearings using the radioblvd.com site only to find the bearings are either sealed or permanently covered, the ball bearings were not visible or accessible.  Before removing the armature I was able to manually spin it so the 70 year old grease hadn’t hardened at least.  I dribbled some Mobile 1 into the front and back of both bearings as that was the only thing I could do.  

There are replacement bearings out there?

I lightly tapped the bearings/armature out of the end caps with no difficulty.  Reassembly seemed fine.  I reinstalled the bearings into the end caps, attached the end caps to the main housing, then lightly tapped on one end (use a socket) until both sides appeared equal distant.  Placed the shim in on each side, then the gasket and cover.  The armature was free wheeling until I put the bushings back in.  Is there more intricate detail that I neglected?  

No I’ve never heard of the military inverters that you mention.  At this point I’m thinking of building either an internal or external AC power supply, solid state, and keep the voltage under 200 Vdc.

I had gotten antsy and turned the BC-348 on, and was met with audio, I tuned in 3 or 4 stations, one at 1500 KC AM (KSTP) and a couple shortwave AM broadcasters above 40 meter.  I couldn’t hear much because there so much interference.  I powered the DM28 from a 28 volt monster switching power supply.  I am assuming that I’m hearing the switcher.  I am going to connect a 28 Vac transformer to the 28v input, and the HV at 170Vdc with the Heathkit HV power supply and see if the RF racket if from the receiver or dynamotor, or the switcher.  

I have the replacement capacitors at the ready.  

73, Scott WA9WFA

From: Michael Hanz <aaf-radio-1 at aafradio.org> 
Sent: Wednesday, April 8, 2020 11:11 AM
To: whitebear1122 at comcast.net; boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] DM-28 Dynamotor Restoration Questions

Thanks for the update, Scott.

Good work on the brush repair!  

On the dyno noise, is this with the dyno inside the receiver and buttoned up in the rear cabinet?  I've never been bothered by mine - the command sets are noisier because the dynos are out in the open, but the 224/348, not so much because of the sealed case.  Did you replace the bearings?  Just curious.

If it is the normal dynamotor noise that bothers, I have modified a couple of the command set solid state DY-10 and PP-5792/ARN-30 inverters to fit on an adapter plate for the BC-348.  One of the guys that worked on the WWII radio sets in the B-29 Fifi was blowing out DM-28s like they were popcorn - something about the flaky primary power system on the aircraft, not the receiver load.  I sent him one of the inverters and a mounting plate (you need to put a 1N5363 30v 5W zener in series to get the voltage down) and last I heard, it solved the problem. 

- Mike  KC4TOS

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